Wind Blowing Up
Hair moves according to where and how strong the wind is blowing. In this instance, the wind is blowing up as indicated by the arrow.
The first example all the way on the left show wind blowing strongly causing the hair to go up pretty much straight. The other example to it's right, though, shows a gently blowing wind. Notice the hair is more wavy and poofy rather than straight and thin.
The way hair is drawn is affected by what the character is wearing. The example all the way on the top right, for instance, has a ponytail. When wind is blowing, the band holds part of his hair down and the rest is being blown. Also, don't forget to draw bangs affected by the wind!
This example on the right basically shows how short hair is affected by wind.
Wind Blowing Around
Wind, in this section, is moving around a character's head, blowing right. The same rules will apply to wind blowing to the left as well.
The first example at top shows hair being constrained from moving too freely with a circlet. On the other hand, the other example on top shows hair moving freely according to the winds direction. Again, remember to draw your bangs moving as well!
For short hair, the example to the right shows strong wind blowing that even the bangs are greatly affected by it! Some people prefer to seperate the bangs and the hair behind it while others blend the two together. How you draw your character's hair depends on you in the end.
Wind Blowing Directionally
If you didn't notice in the other two pages, drawing hair will be the same in this case as well - this time, the character's are just on a side view. To recap, hair flows freely but if something is keeping it down, make sure you draw your hair accordingly.
If the wind was blowing in the same direction as the character was facing, the hair would be covering the face. The longer-haired guy would also have his hair wrapping around his neck and jaw.
Good Ole Gravity
Of course, you can't have hair movement if you don't have gravity included! Most hair, when no wind is blowing, will have a downward direction - duh! Just remember, hair can be moved by the character's shoulder when bending down or the like.
Oh yeah, when there is no gravity, long hair just floats. You know, like the wavy hair up top? Except more wavy and spread out than just going up.